I am beginning to mock up and measure for my VW-based trike. At present I plan to use a '79 HD Sportster frame since I have a clear title in my name for it. I've crudely set it up at about the height it will be when the torsion housing is added. The steering head rake can vary from about 29 degrees to about 35 degrees. I plan for the torsion housing to mount to the end of the swing arm and solid links will replace the Sportster shocks. My inquiry regards the fork tube length.I plan to use a HD Wide Glide 5 degree rake triple tree and 41mm tubes with a 21" front wheel. My question is how do I determine how long the fork tubes need to be other than by trial and error? Overall length from top of tube to center of axle is easy enough but how do I determine the length of just the tubes minus the lower legs?